The electrophotographic method of forming a visual image by developing an electrostatic latent image with a developer includes forming an electrostatic latent image on a photoreceptor including photoconductive material, forming a toner image by developing the electrostatic latent image with a developer including a toner, transferring the toner image onto a recording medium such as papers, and forming a fixed image thereon by fixing the toner image with heat and pressure.
The toner is typically a colored particulate material formed of a binder resin including a colorant, a charge controlling agent and other additives, and is mostly prepared by a pulverization method or a suspension polymerization method. The pulverization method includes melting, mixing and dispersing a colorant, a charge controlling agent, etc. in a thermoplastic resin to prepare a composition; and pulverizing and classifying the composition to prepare a toner.
In order to save energy and downsize a toner, which is difficult for the pulverization method, chemical toners prepared by the suspension polymerization method, an emulsion polymerization method, a dissolved resin suspension method, etc. are becoming popular.
A toner set which is a combination of a cyan toner, a magenta toner, a yellow toner which are three-color process toners and a black toner is typically used to form a full-color image by the electrophotographic method.
A developing order of the toners when forming a full-color image is not limited, but e.g., light from a document is irradiated on a photoreceptor through a color separation filter or an image read by a scanner is written with a laser irradiation on a photoreceptor to form an electrostatic yellow latent image thereon.
The electrostatic yellow latent image is developed with a yellow toner to form a yellow toner image, and which is transferred onto a recording medium such as papers.
Next, a magenta toner image, a cyan toner image and a black toner image which are similarly prepared with a magenta toner, a cyan toner and a black toner, respectively are sequentially overlapped on the yellow toner image to form a full-color image.
However, as the electrophotographic full-color image forming apparatuses become widely used, their applications multifariously expand and demands for their image quality are becoming more severe.
Copies of pictures, brochures and maps are required to very finely and faithfully reproduce the original images. Demands for color brightness are becoming high as well and color reproduction range is desired to expand.
Recently, even the electrophotographic image forming methods have been required to produce images having high-definition as good as or better than that of printed images.
Japanese published unexamined application No. 2000-343476 discloses a method of using a highly-colored toner for solid part and a low-colored toner for highlight part besides the three-color process toners to form an image.
Japanese published unexamined applications Nos. 2004-118020 and 2004-142153 disclose a method of reproducing delicate color tone using seven color toners including additional red, blue and green toners.
However, multicolor toners increases cost of preparing the toners and enlarges the apparatus, and it is reasonable to increase only one color. Japanese published unexamined applications Nos. 2007-304401 and 2011-008079 discloses an orange color toner, which is difficult for the three-color process toners to reproduce. However, improvement of blue color reproducibility which is difficult for the three-color process toners to reproduce as well is not expected.
Because of these reasons, a need exist for a four-color process toner set producing images having maximum color reproducibility with only one additional color toner.